NotebookLM (notebooklm.google.com — free with a Google account) lets you upload up to 50 sources per notebook. Then you ask questions across all of them at once.
It will not invent answers. Every response cites which source it pulled from. This is called a constrained query — the AI is locked to your material. No hallucinations. No outdated training data. Just your stuff, organized and searchable in plain English.
Open NotebookLM